As an alternative to typographic and printing techniques so far generally available in the art, ink jet, heat transfer and other systems have been developed to make excellent monochromatic or full-colored images in simpler and faster manners. The most advanced of all is the so-called sublimation heat transfer system making use of a sublimable dye to give full-colored images of excellent continuous gray scale comparable to color photographs.
In general, heat transfer sheets used with the sublimation type heat transfer system each include a substrate film such as a polyester film, having a sublimable dye-containing dye layer formed on one surface and a heat-resistant layer provided on the other surface so as to prevent it from sticking to a thermal head.
Such a heat transfer sheet is overlaid at the surface of its dye layer on an image-receiving sheet containing an image-receiving layer made of a resin such as polyester, and is then heated from its back surface in an imagewise form with a thermal head, thereby transferring the resin from the dye layer onto the image-receiving sheet to form the desired image.
The heat transfer system is advantageous in that shading levels of an image can be determined by increasing or decreasing the temperature of a thermal head. A problem with this technique, however, is that as the temperature of the thermal head is elevated to increase the density of the image, the dye layer-forming binder softens to such an extent that it adheres to the image-receiving sheet, causing the heat transfer sheet to bond to the image-receiving sheet or, at worst, the dye layer to be transferred from the substrate film immediately onto the image-receiving sheet at the time of releasing.
In order to solve such a problem, it has been proposed to incorporate a release agent such as silicone oil in the dye-receiving layer of the image-receiving sheet. A problem with this proposal, however, is that due to being liquid at normal temperature, the silicone oil tends to bleed through the dye-receiving layer, posing blocking and contamination problems. Use of a heat curable silicone oil, on the other hand, has been envisaged. Required to this end, however, are heat treatments after the formation of the dye-receiving layer, which make manufacturing steps very troublesome.
Imparting sufficient release properties to the dye-receiving sheet may also be achieved by the addition of a relatively large amount of silicone; however, this will result in a drop of dye receptivity and a degradation of the storability of the dye-receiving layer.
Also, when a transparent film is to be laminated on the surface of the resulting image for its protection, it would be difficult, if not possible, to achieve satisfactory lamination, because the image layer contains a release agent.
Addition of a release agent to the dye layer of a heat transfer sheet in an amount nothing short of imparting some release effect to it, on the other hand, will result in dye bleeding or discoloration, or make it unusable due to a drop of its storability.
It is, therefore, an object of this invention to provide a heat transfer sheet which can be produced in a much simpler manner, enables an image of high density to be formed at high speed with the prevention of a drop of the thermal migration of a dye and with neither adhesion between the dye layer and the dye-receiving layer nor peeling of the dye layer at the time of heat transfer, and renders it possible to make an image on the surface of which a transparent film can be laminated.